This application addresses the broad Challenge Area (15) "Translational Sciences and specifically the challenge topic 15-DK-106 : Translating basic hematology concepts". Diamond Blackfan anemia is a rare bone marrow failure syndrome characterized by red cell aplasia, congenital abnormalities and a predisposition to cancer. Heterozygous mutations in ribosomal protein genes have been found in approximately 50% of DBA patients. As all patients identified to date are heterozygous for these mutations, haploinsufficiency has been suggested as the mechanism underlying the pathogenesis of DBA. However, the relationship between ribosomal protein deficiency and defective erythropoiesis is not known. Work from several groups has demonstrated an intriguing connection between ribosomal proteins, specifically RPL5 and RPL11, and the regulation of p53. In human cell lines with a wild type p53 response, depletion of ribosomal proteins causes cell cycle arrest and induction of p53 and p21. Interestingly, this response and cell cycle arrest can be abrogated by simultaneous depletion of either p53 or RPL11. From this, we hypothesize that DBA is caused by disrupted ribosome biogenesis causing an imbalance of ribosomal proteins that, by inhibiting MDM2, causes an activation of the p53 pathway that is detrimental to the maturing red cells. We further hypothesize that modulation of this downstream pathway may correct the erythroid defect. One factor that has greatly limited investigations into the pathogenesis of DBA is the absence of a model that recapitulates the hematopoietic defects seen in DBA patients. Therefore we have generated a mouse model that closely mimics DBA hematopoiesis. We now propose to test our hypotheses and further characterize the block in red cell differentiation in these mice. We will rescue erythropoiesis in vitro and subsequently in vivo by manipulating the levels of key components shown to link defective ribosome biogenesis to the activation of p53. These experiments will provide important new insights into how ribosome biogenesis regulates cell proliferation in vivo. Moreover these experiments approach the major puzzle about DBA pathogenesis, namely how does haploinsufficiency of a housekeeping gene cause a defect in one specific cell type to produce red cell aplasia in DBA? Additionally, an animal model mimicking the hematopoiesis of patients with DBA which can be rescued in vitro and in vivo will be invaluable for developing and testing novel drugs that may eventually improve the treatment of DBA and possibly other diseases caused by ribosomal defects. PUBLIC HEALTH RELEVANCE: Diamond Blackfan anemia is a genetic blood disorder that manifests with a severe anemia in childhood. The disease is caused by failure to make red blood cells due to problems in the protein synthesis machinery. To study the disease mechanism in more detail we propose to generate a mouse model that has the same genetic defect and the same problems in red blood cell production as patients with DBA. We plan to investigate the defect in red blood cell production in these mice in great detail and test whether interference with the pathway that causes the problem will rescue red blood cell production. These studies may lead to greater understanding of DBA and of ways we might treat this disease.